Mode division multiplexing (MDM) technology is a potential next-generation solution to improve the capacity of optical access networks in a cost-effective way and to provide backward compatibility with legacy standard single-mode fibre optic networks. In theory, an N-fold capacity increase can be obtained by using a few-mode fibre (FMF) to guide N independent modes. However, there are two effects seen in FMFs which impair the signal and need to be addressed in order to reach full capacity. These effects are (i) linear modal coupling (crosstalk), and (ii) differential mode delay. On long distance applications, the interplay between these effects typically requires the use of a coherent receiver in order to enable their mitigation through digital signal processing (DSP).
The basic architecture 100 of a known MDM passive optical network (PON) for supporting 6 modes (LP01, LP11a, LP11b, LP21a, LP21b, LP02) is shown in FIG. 1. At an input (transmitter) side there are six optical line terminations (OLTs) 102, which are located in the same facility (often referred to as a Central Office (CO)). At an output (receiver) side, there are six optical network units (ONUs) 104, which are typically distributed in different physical locations.
Each OLT is connected to a transmitter side mode multiplexer 108 by a respective single mode fibre (SMF) 106. The mode multiplexer combines the signals from the OLTs 102 and transmits them on a few mode fibre (FMF) 110. At the receiver side, a mode demultiplexer 112 extracts each relevant signal and outputs to each respective ONU 104 via a respective single mode fibre 114.
The system in FIG. 1 introduces new impairments to the transmitted signal that are not encountered in single mode fibre passive optical networks, namely:                the mode multiplexer 108 and mode demultiplexer 112 can introduce a non-negligible amount of crosstalk;        the FMF 110 can introduce different differential mode delay and different crosstalk levels between different pairs of linearly polarized (LP) modes.        
For pairs of non-degenerate LP modes, such as LP01 and LP11a or LP01 and LP11b, the crosstalk strength can be as low as −40 dB/km (e.g. −27 dB at the end of 20 km), but the differential mode delay can be as high as 1000 ps/km. In contrast, for pairs of degenerate LP modes, for example LP11a and LP11b or LP21a and LP21b, the crosstalk strength is much higher such that full mixing can be achieved after a couple of tens of kilometres but the differential mode delay can be lower than 1 ps/km.
The different effects for degenerate and non-degenerate modes can be understood by considering the transfer matrix for the FMF 110. A FMF can be modelled as N sections, where each section is modelled by one unitary matrix XT introducing the crosstalk and one diagonal matrix DMD whose diagonal elements introduce the mode delay. FIG. 2 shows an example of these matrices for the ith section of an FMF.
In general, the fibre matrix HFMF(ω)=XT1DMD1 . . . XTNDMDN is dependent on the frequency whenever the differential mode delay is non-negligible. Since this is the case in general for pairs of non-degenerate LP modes, the full fibre matrix is dependent on frequency.
In known MDM techniques, channel estimation/inversion is usually done in the electrical domain after detecting the modes all together. However, in the architecture described in FIG. 1, the modes are detected independently, which means it is impractical or impossible to use a DSP at the receiver end, e.g. because it would require replacement of an already-installed SMF between the mode DEMUX and the customer premises and installation of separate DSP-capable ONUs at each customer premises. For these reasons, it is desirable for channel estimation/inversion to be done at the transmitter end if this detection technique is to be used.
Channel estimation at the CO requires the communication/cooperation between OLTs (enabled by the backplane) and the downstream transmission of training sequences or pilot signals (which must be different for each mode), which have to be retrieved by the transmitter somehow. In this way, different OLTs will receive different combinations of the training sequences or pilot signals, which when combined allow for the estimation of the channel matrix and consequent pre-compensation. However, a disadvantage of this arrangement is that the training sequences/pilot signals will experience crosstalk on the upstream transmission during retrieval by the transmitter. In this scenario, it becomes very difficult to estimate the downstream fibre matrix.